Aromatic anhydride compounds have well established utility in the preparation of a wide variety of both thermoplastic and thermoset compositions. For example, polyimides are prepared via the condensation reaction of aromatic dianhydrides of tetracarboxylic acids with diamines. Typical of the dianhydride reactants employed are benzophenone tetracarboxylic dianhydride, pyromellitic dianhydride, 1,2,3,4-butanetetracarboxylic dianhydride, methanetetraacetic dianhydride, cyclooctadienetetracarboxylic dianhydride and pyrazinetetracarboxylic dianhydride.
Additionally, aromatic anhydride compounds have found utility as curing agents for epoxy resins. Products have heretofore been prepared by curing diglycidyl ethers which contain a mesogen selected from the group consisting of the direct bond, --CH.sub.2 --CH.sub.2 --, --CH(CH.sub.3)--CH.sub.2 --, --CH(CH.sub.3)--CH(CH.sub.3)--, --CH.dbd.CH--, --N.dbd.N--, --CH.dbd.C(CH.sub.3)--, --CH.dbd.N--, --O--CH.sub.2 --, and --S--CH.sub.2 -- linkage between a pair of substituted or unsubstituted benzene rings with non-mesogenic anhydride curing agents which are disclosed by German Patent No. 4021328. A single anhydride curing agent, biphenyltetracarboxylic dianhydride, disclosed therein contains the biphenyl group, a mesogenic moiety, however, the direct tetrasubstitution on the pair of aromatic rings of the biphenyl group by the pair of anhydride groups precludes liquid crystallinity. This is because anhydride ring opening, as occurs during the curing reaction between the epoxide and anhydride group provides a pair of linkages between each aromatic ring of the biphenyl and the epoxide curing structure which are ortho to each other. It is well established that for liquid crystallinity to occur, para substitution must predominate.
It would be desirable to have available curing agents which could provide improved properties resulting from curing epoxy resins which do not contain mesogenic moieties and are readily available. It would also be desirable to have available epoxy resin curing agents which contain mesogenic moieties for use in curing epoxy resins which also contain mesogenic moieties. The simultaneous presence of mesogenic moieties in the epoxy resin as well as the curing agent allows for a greater concentration of said moieties in the cured products thereof and thus a higher susceptibility to induced orientation and the enhanced mechanical properties resulting therefrom. It would also be desireable to have available mesogenic dianhydride curing agents for epoxy resins in which the anhydride groups are not substituted directly on the aromatic rings of the mesogen, but rather, are coupled through the mesogen via some type of flexible spacer or linkage via para substitution to the mesogenic moiety.
The present invention provides a heretofore unknown class of aromatic anhydride compounds which contain rodlike mesogenic moieties and thus can be used to provide cured epoxy resin products possessing an improvement in one or more physical and/or mechanical properties. Said aromatic anhydride compounds are para substituted via a spacer group between the anhydride groups and the mesogen and thus provide the desired liquid crystalline character.